The present invention relates to lighting devices, and more particularly to improvements in optical delivery systems suitable for use in dental and medical operatory lights.
A dental and medical operatory light differs significantly from an ordinary light projector by its ability to provide cool and color-corrected uniform illumination approaching pure white light comparable to sunlight in the 5000.degree. K to 6000.degree. K color temperature region with a color rendering index of greater than 90, and over a sharply-defined target area. Color correction is especially necessary in dentistry for distinguishing the shades of color presented by natural teeth which must be color-matched with artificial teeth. Natural teeth present a problem of translucency where internal layers of a tooth such as the enamel and dentin, make matching the colors of a difficult and complex exercise. Color-corrected light helps the dentist make a more perfect match. Obtaining natural corrected color also helps the professional in diagnosing soft tissue diseases.
The operatory light generally comprises at least one optical delivery system of a high intensity light source and with reflectors, and still others include lenses and filters with optical aperture stops for producing a sharp beam of light at a correlated color temperature approximately between 5000.degree. K and 6000.degree. K. A more detailed explanation of the optical principles involved is discussed in U.S. Pat. No. 4,608,622 for "Multi-Function Light Source" by D. I. Gonser.
Ideally, the optical delivery system should faithfully reproduce all of the colors at the target area, but in practice a variety of aberrations occur which are attributable to the optical delivery system, particularly to the lenses. Several types of aberrations are spherical aberration, astigmatism and curvature of field, distortion, and lateral and longitudinal (axial) chromatism. In dental operatory lights, chromatic aberration is probably the most detrimental of these because it produces a visible spectrum with a hue projected on the illuminated target. Hue tends to interfere with the ability to see slight differences in shades of color, particularly on white or near white surfaces. Shade matching with the naked eye is much more accurate under white light.
The origin of hue is caused by variations in the index of refraction of the lenses with wavelength. The shorter the wave length, the less diffraction. For example, the refractive index is higher for blue light than for red. Consequently, the focal point of the blue component is closer to the lens than the red component resulting in a purple hue on the illuminated surface. So if white light is incident on a lens, the different colors are focused at different points and result in colored fringes or so-called hue on a target area. These color hues cause erroneous or imperceptible differences to the naked eye on nearly white surfaces.
Presently there does not exist a simple and economic color-correcting optical delivery system for use in the relatively confined space of a dental or medical operatory light which eliminates hue by cancellation of chromatic aberrations.